Devices such as hot water heaters, furnaces, and other appliances commonly include one or more heating elements that are controlled by a controller such as a thermostat. A heating element is activated (i.e., placed in an on-state) when heat is needed and deactivated (i.e., turned to an off-state) when heat is not required. The change of states normally occurs when a control signal turns a power relay on or off. Power relays have a pair of contacts capable of meeting the current requirements of the heating element. In a typical home-use hot water heater, approximately 220 volts AC is placed across the heating element and a current of about 10 to 20 amperes flows.
A heating element is typically associated with an upper temperature threshold, referred to as the “upper set point,” and a lower temperature threshold, referred to as the “lower set point,” that are used for control of the heating element. When the temperature of water in a tank exceeds the upper set point, as measured by a thermal sensor mounted on a wall of the water heater, the heating element is deactivated, and heating of the water by the heating element stops. If the water temperature drops below the lower set point, the heating element is activated and, therefore, begins to heat the water. As heated water is repeatedly withdrawn from the water tank and replenished with cold water, the heating element goes through activation/deactivation cycles.
One problem associated with water heaters is “stacking” wherein water in the upper section of the tank reaches high temperatures that are significantly greater than the upper set point and often much higher than expected by a user. Because a hot water supply pipe of a water tank typically draws water from the top of the tank, stacking may cause the water drawn from the tank to significantly exceed the upper set point. Such an undesired effect can result in pain or injury to a user that touches the overheated water coming from the hot water supply pipe.
Thermal lag can also cause water within the tank to become overheated. “Thermal lag,” as used herein, refers to a delay in the temperature of the water reaching the upper set point and a detection by the thermal sensor that the upper threshold has been reached. Thermal lag can cause water temperature to overshoot the upper set point value and, therefore, reach undesirably high levels. Hence, there is a need for reducing undesirable overheating of water within a water heater due to stacking and thermal lag.